Field of the Invention
The present invention relates to an image communicating apparatus, a control method therefor, and a storage medium storing a program.
Description of the Related Art
In conventional facsimile (FAX) transmission/reception, digital image data is modulated into an analog signal by a modem, and transmitted via a PSTN (Public Switched Telephone Network), and an analog signal received from the PSTN is demodulated into digital data, and then received.
To the contrary, in recent years, FAX transmission using a high-speed network such as an IP network is becoming widespread. There are two methods for FAX transmission using the IP network. One of the methods is a transmission method called T.38 of ITU-T recommendation, in which a digital signal transmitted according to the T.30 protocol is transmitted intact, and which can implement high-speed transmission, as compared with T.30. The other method is deemed voice transmission using the T.30 protocol of ITU-T recommendation. This method is a method of performing transmission by modulating data into an analog signal by a modem, encoding it by a voice codec, and adding an IP header to form the encoded signal into an RTP packet. This method is called deemed voice transmission since a T.30 analog signal is transmitted by deeming it a voice signal.
In the conventional FAX transmission/reception, since a modem modulates a digital signal as transmission data into an analog signal, and demodulates an analog signal as reception data into a digital signal, a data I/F between the modem and a line is an analog I/F. In deemed voice transmission, however, a digital signal as transmission data is modulated into an analog signal, and formed into a voice packet by a voice codec. Alternatively, since it is necessary to decode a voice packet as reception data by a voice codec, and demodulate the decoded data into a digital signal, a digital modem which incorporates a voice codec and can input/output a digital signal is used. Note that ITU-T recommendation G.711 is used as a voice packet encoding method and RTP (Real Time Protocol) is used as a transmission protocol. The above-described two methods implement Point-to-Point transmission on the IP network using SIP (Session Initiation Protocol) as a call control unit. Examples of the IP network are a digital public network called an NGN (Next Generation Network) provided by NTT, and a private IP line using an IP private branch exchange (IP-PBX) or the like. Especially in the case of the NGN network, for example, the bandwidth is guaranteed and the security is managed, thereby allowing high-speed secure image transmission. Although the transmission rate of the conventional PSTN is at most 33.6 Kbps, the NGN guarantees a transmission rate of at most 1 Mbps, thereby allowing transmission at a transmission rate about 30 times higher than that of the PSTN.
On the other hand, an IP phone is a technique of implementing voice communication by encoding a voice into PCM data, and transmitting/receiving it on the IP network. Using this technique can implement voice communication by the handset/slave (to be referred to as a handset hereinafter) of a FAX. An SLIC (Subscriber Line Interface Circuit) for connecting a telephone and the IP network is used to connect the handset to the IP network. The main application purpose of the SLIC is encoding of an analog voice into a voice packet, detection of a hooking operation of the telephone, identification of a dial signal from the telephone and the like, ringing control of the telephone, and the like.
By setting a mode, a conventional facsimile apparatus connected to an analog network has a function (FAX/TEL switching) of automatically switching between a telephone and a FAX or a function (message recording/TEL switching) of automatically switching between a message recording device and a FAX.
In FAX/TEL switching, when the FAX receives a calling signal from an exchange a predetermined number of times, it terminates a call, and the FAX sends a pseudo ringback tone (pseudo RBT) imitating a ringback tone while calling a slave connected to itself. Alternatively, the FAX sends a message (OGM) like “Now calling. Please hold on. Please send in the case of a facsimile.” If a CNG signal (a 1100-Hz tone) from a transmitter is detected during this time, FAX reception starts. If no CNG signal can be detected, the FAX continues calling the slave. When the slave is hooked up, voice communication using the telephone starts.
The modem performs all of such sending of a pseudo RBT signal or OGM signal and detection of a CNG signal. Since, however, the modem can perform only one process at once, it cannot detect a CNG signal while sending an OGM signal or pseudo RBT signal.
Conventionally, a silence period of several sec during which neither a pseudo RBT signal nor an OGM signal is sent is provided after a receiver terminates a call. An attempt is made to detect a CNG signal from a transmitter during the silence period, and a CNG signal is detected during the regular silence periods at the time of sending of a pseudo RBT. For example, Japanese Patent Laid-Open No. 9-252364 describes a technique of detecting a CNG signal by causing an analog signal to pass through an RBT block filter. If, however, a CNG signal is received during a period other than the silence period, the CNG signal cannot be detected, and is missed in some cases.
An deemed voice IP FAX connected to the IP network has the same problem, and it is impossible to detect a CNG signal while sending a pseudo RBT signal or OGM signal by a digital modem. If a CNG signal is received but cannot be detected, a slave rings before a transmitter transmits a CNG signal next time, and the user off-hooks the slave. This disables FAX reception. In addition, during the silence period of several sec in which neither a pseudo RBT signal nor an OGM signal is sent, a sender becomes suspicious and disconnects a call.